Imaging array and methods for fabricating same

ABSTRACT

A radiation detector includes a first array including a first photon incident surface, a second array including a second photon incident surface, and a scintillator array extending from the first photon incident surface to the second photon incident surface. The second detector offsets from the first detector by approximately one-half detector pitch normal to an incident x-ray direction.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 10/308,233 filed Dec. 2, 2002.

BACKGROUND OF THE INVENTION

[0002] This invention relates generally to radiation imaging systems andin particular to x-ray radiography imaging systems.

[0003] Imaging arrays typically include a photosensor array coupled to ascintillating medium. Radiation absorbed in the scintillator generatesoptical photons which in turn pass into a photosensor, such as aphotodiode. To increase resolution, some known imaging systems utilize adynamic focal spot wobble technique which increases a computedtomography imaging system resolution by manipulating the position of anx-ray focal spot during data acquisition. Other known imaging systemsincrease a resolving power of the imaging system by combining projectiondata that are scanned 180 degrees apart. At least one known imagingarray includes a photodiode panel with a pitch size of approximately 100microns that can only achieve a resolution as high as 5 lp/mm (linepairs per millimeter) from a single measurement due to limits set by thesampling rate and the corresponding Nyquist frequency. For example, insignal processing, an ideal detector with an aperture size of d has afrequency resolving power up to 1/d in a Fourier domain before itsModulation Transfer Function (MTF) curve of a SINC function hits itsfirst zero node. At least one known linear array detector with a pitchsize of d can only resolve a spatial frequency up to the Nyquistfrequency 1/2 d, if only one measurement is taken.

BRIEF SUMMARY OF THE INVENTION

[0004] In one aspect, a radiation detector is provided. The radiationdetector includes a first array including a first photon incidentsurface, a second array including a second photon incident surface, anda scintillator array extending from the first photon incident surface tothe second photon incident surface. The first array offsets the secondarray by one-half detector pitch normal to the incident x-ray direction.

[0005] In another aspect, a method for fabricating a radiation detectoris provided. The method includes fabricating a first array including afirst photon incident surface, fabricating a second array including asecond photon incident surface, and positioning the second array with anoffset of half of the detector pitch normal to the incident x-raydirection, and positioning a scintillator array between the first arrayand the second array such that the scintillator extends from the firstphoton incident surface to the second photon incident surface.

[0006] In a further aspect, an x-ray imaging system, e.g. projectionradiography or a computed tomography (CT) for generating an image of anobject with high resolution is provided. The imaging system includesdetector array and one radiation source, object scanning system(rotation or linear motion), and a computer coupled to the detectorarray, scanning mechanism, and the radiation source. The detector arrayincludes a first array of detector pitch (d), and a second array ofdetector pitch (d) equivalent to the first detector pitch, the seconddetector offset from the first detector by approximately one-halfdetector pitch normal to the incident x-ray direction. The computer isconfigured to sample the first array and the second arraysimultaneously, and reconstruct an image of the object with higherresolution by interleaving data of the first array samples and thesecond array samples.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is an imaging system including an x-ray source, object, andan x-ray detector.

[0008]FIG. 2 is a side-sectional view of the detector shown in FIG. 1.

[0009]FIG. 3 is a cross-sectional view of the detector normal to theincident x-ray direction shown in FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

[0010]FIG. 1 is an imaging system 10 including an x-ray source 12 and anx-ray detector 14. FIG. 2 is a cross-sectional view of detector 14 shownin FIG. 1. FIG. 3 is a cross-sectional view of detector 14 normal to theincident x-ray direction shown in FIGS. 1 and 2. In an exemplaryembodiment, imaging system 10 is a computed tomography (CT) imagingsystem which the object can be rotated during measurement. X-raydetector 14 includes an array 16 of sensor elements 18. Detector 14 isdisposed to receive incident x-rays that have passed through an object20 that is to be imaged. In one embodiment, imaging system 10 includes acollimator 22 disposed such that x-rays exiting from object 20 passthrough collimator 22 before striking x-ray detector 14. The lightgenerated by the absorption of incident x-rays detected by x-raydetector 14 generate an electrical signal corresponding to the incidentx-rays.

[0011] In an exemplary embodiment, detector 18 includes a firstphotosensor array 30 including a first photon incident surface 32, asecond photosensor array 34 including a second photon incident surface36, and a scintillator array 38 extending from first photon incidentsurface 32 to second photon incident surface 36. The light generated bythe absorption of incident x-rays in scintillator array 38 is detectedby first photosensor array 30 and second photosensor array 34. Firstphotosensor array 30 and second photosensor array 34 each generate anelectrical signal corresponding to the incident x-rays detectedrespectively.

[0012] In an exemplary embodiment, sensor element 18 includes aplurality of photosensor devices 40, such as, but not limited to aplurality of photodiodes 40. In one embodiment, each sensor element 18includes sixty-four photosensitive diodes. Each respective sensorelement 18 generally has the same number of photosensitive diodes. Inone embodiment, photosensor devices 40 are electrically coupled togetherin a parallel circuit arrangement.

[0013] In one embodiment, photosensors 40 are thin film semiconductorsfabricated from at least one of a hydrogenated amorphous silicon, ahydrogenated amorphous germanium, a fluorinated amorphous silicon, afluorinated amorphous germanium, an alloy including silicon andgermanium, and a polycrystalline silicon. Deposited, as use herein,refers to the fabrication of the semiconductor device through successivedepositions forming large thin film semiconductor devices. For example,the materials can be deposited in a chemical vapor deposition processand patterned to form the desired components in an array.

[0014] In one embodiment, sensor elements 18 include hydrogenatedamorphous silicon, hereinafter referred to only as amorphous silicon ora-Si, and are fabricated using known technology for forming large areathin film arrays. Using amorphous silicon for fabricating firstphotosensor array 30 and second photosensor array 34 facilitatesfabricating a large array of respective relatively small diodes on asingle substrate, this arrangement facilitates reducing packagingproblems and provides an effective large active area responsive toincident light. This arrangement further facilitates providing for aplurality of diode structures in a compact area.

[0015] Using amorphous silicon also facilitates reducing common defectsin first photosensor array 30 and second photosensor array 34, such as,but not limited to, short circuits between conductive components thatare readily repaired with laser ablation techniques. Using amorphoussilicon also facilitates reducing damages since X-ray radiation does notdamage amorphous silicon devices as much as it damages single crystaldevices. As described herein using amorphous silicon facilitatesimproving both the performance of detector 14 and the efficiency ofarray fabrication.

[0016] By way of example, and not limitation, in one embodiment detector14 is used for industrial CT purposes (e.g., imaging turbine parts), andincludes 2048 focally aligned sensor elements 18 in each firstphotosensor array 30 and second photosensor array 34. In one embodiment,sensor elements 18 include a pitch of approximately 100 microns (μm),i.e., separation between adjacent sensor elements. The correspondingsensor elements 18 between the first photosensor array 30 and the secondphotosensor array 34 have an offset of approximately fifty micronsnormal to an incident x-ray direction. Larger or smaller numbers ofsensor elements 18 can be used in detector 14 depending upon theparticular use for imaging apparatus 10. In one embodiment, sensorelement 18 includes sixty-four separate diodes 40, wherein each diode 40is approximately 100 μm in width and approximately 500 μm in length. Inone embodiment, sensor elements 18 are approximately 32 millimeters (mm)in length. Sensors 18 are disposed along a respective focal axis 44 andhave a length of approximately 32 mm to facilitate absorbing the x-raysemitted from source 12. In one embodiment, source 12 operates at avoltage of approximately 200 kilovolts or greater. In anotherembodiment, source 12 operates at a voltage between approximately 200kilovolts and approximately 700 kilovolts.

[0017] In use, components of imaging system 10 are arranged such that aplurality of x-rays 50 emanating from x-ray source 12 are directed to beincident on object 20 to be imaged. X-rays that pass through object 20represent the object due to the relative attenuation of the x-rayspassing through different portions of object 20. For purposes ofillustration only, an x-ray attenuation pattern 52 of a region ofinterest of object 20 represents those x-rays that have passed throughobject 20. Attenuation pattern 52 is typically a function of thestructure, variations in thickness, and variations in material types ofobject 20. Attenuation pattern 52 as used herein, describes a spatialvariation in x-ray intensity due to absorption or scattering of incidentx-rays by object 20.

[0018] In one embodiment, slit collimator 22 is disposed between x-raysource 12 and detector 14 such that, upon exiting collimator 22, thex-rays of attenuation pattern 52 are incident on scintillator 38.Collimator 22 thus determines the slice resolution of imager apparatus10. In one embodiment, collimator 22 includes a material that is opaque,such as, but not limited to, tungsten. Collimator 22 includes a slit 54,i.e. a narrow opening in collimator 22. In one embodiment, slit 54includes a height between approximately 25 μm and approximately 1000 μm.In another embodiment, slit 54 includes a height between approximately50 μm and approximately 250 μm, and a length comparable to a width ofsensor array 16. In another embodiment, imaging system 10 includes asecond collimator (not shown), disposed between x-ray source 12 andobject 20.

[0019] In use, a portion of the x-rays passing through object 20 alsopass through slit 54 of collimator 22 and then pass into scintillator38. The x-rays are absorbed in scintillator 38 in events that result inthe generation of optical photons. Scintillator 38 is disposed betweenfirst photosensor array 30 and second photosensor array 34, and extendsfrom first photon incident surface 32 to second photon incident surfacesuch that the light generated in scintillator 38 is optically coupled tofirst photosensor array 30 and second photosensor array 34. Thus, thex-ray pattern 52 representing object 20 is converted to visible lightwhich in turn impinges upon first photosensor array 30 and secondphotosensor array 34 in detector 18. The incident light is converted toan electrical signal that is representative of the light absorbed.(e.g., by accumulation of charge on respective diodes) by thephotodiodes in first photosensor array 30 and second photosensor array34. Thus, ray pattern 52 is sampled with a detector element 18 pitchsize of approximately d and a step size of approximately d/2 whencombining the samples from the first photosensor array 30 and secondarray 34 together to facilitate providing an increase in the detectorresolution.

[0020] The accumulated charge on the diodes is read out by an array ofamplifiers 56. In one embodiment, each sensor element 18 in firstphotosensor array 30 and second photosensor array 34 includes a singleamplifier 56. In one embodiment, amplifiers 56 include chargeintegrating amplifiers or alternatively current to voltage amplifiersfollowed by an integrating stage. The total charge incident on eachsensor element 18 during any desired period of time is sampled usingamplifiers 56 and the resulting data transmitted to a data processor 58for presentation or further computation.

[0021] The data generated, from one position of slit collimator 22 withrespect to object 20, represent a view of a single slice of object 20.The data for one view of the single slice are referred to as a frame.For computed tomography, many frames of data are taken at differentangles through object 20 for a single slice. In one embodiment, dataprocessor 58 is programmed to control operation of imaging system 10 forchanging the relative arrangement of object 20 with respect to x-raysource 12 and collimator 22 for obtaining different views. Processor 58is further configured to manipulate the digital data of the collectionof frames into a useful image that is presented on a display 60. Display60 may be an electronic display, a hard copy print out, or any otherkind of display that is visible or otherwise useful to human beings.

[0022] In one embodiment, scintillator 38 includes a plurality ofoptical fibers 61 bundled together. The optical fibers have an opticalaxis 62 that is oriented generally orthogonally to the path of theincident x-rays passing through collimator 22. X-rays absorbed inscintillator 38 are converted to visible light, and the optical photonsgenerated generally pass along respective optical axis 62 of arespective fiber where the absorption took place towards firstphotosensor array 30 and second photosensor array 34, where it isdetected by respective photosensitive diodes. The magnitude of chargeaccumulated in photodiodes 40 is proportional to the intensity of thex-rays passing through slit collimator 22 and that are absorbed inscintillator 38.

[0023] In one embodiment, the optical fibers are fabricated from a fiberoptic light guiding scintillator material, such as, but not limited to amaterial provided by Collimated Holes, Inc. of Campbell, Calif. Thismaterial is typically provided in 4 inch by 4 inch sheets with lengthsof fibers up to 25 mm in thickness corresponding to the dimension “D” inFIG. 2. Pieces of this fiber optic scintillator material can be coupledtogether to form a scintillator 38 that has dimensions correspondingwith a desired arrangement of sensor elements 18. For example, forsensor elements 18 having a length dimension “L” between approximately 1mm and approximately 35 mm, and commonly about of about 32 mm, two ormore pieces of the fiber optic scintillator material are cut andassembled together to cover the desired dimensions of sensor elements18. The depth, that is, the length of the scintillator (dimension “L”)along the direction of the x-ray focal axis of fiber optic scintillatoralong the focal axis (that is, the distance through which an incidentx-ray could travel within the scintillator) is typically in the range ofapproximately 5 mm. Each of the scintillator optical fibers furtherexhibits an emission bandwidth (that is, generates optical photons inresponse to absorption of an x-ray) in a wavelength range ofapproximately 20 nanometers, with the center of that emission bandwidthbeing within a range of wavelengths between about 530 nanometers andabout 550 nanometers.

[0024] In one embodiment, the individual fiber diameters are betweenapproximately 5 μm and approximately 25 mm, providing a large number offibers overlying each photosensor 40 in sensor element 18. In oneembodiment, the fibers are tightly packed together such that adjoiningfibers contact one another, thus letting the fiber diameter effectivelydetermine the number of fibers that can be disposed over the surfacearea of a photosensor 40. In use, the optical fibers direct at least aportion of the light generated from the absorption of incident x-raystowards first photosensor array 30 and second photosensor array 34 andlimits the light from spreading out laterally within scintillator 38since at least a portion of the light is confined to the fibers by totalinternal reflection. In one embodiment, the fibers have a length betweenapproximately 2 millimeters and approximately 10 millimeters. In oneembodiment, the fibers have a length of approximately 6 millimeters. Inanother embodiment, scintillator 38 includes a relative thin sheet ofscintillator material that does not have optical fibers, such as but notlimited to cesium iodide or the like.

[0025] In one embodiment, detector 12 includes a first photosensor arrayand a second photosensor array identical to the first photosensor array.Incident x-rays are collimated to irradiate along the central line ofscintillator 38. Additionally, since first photosensor array 30 andsecond photosensor array 34 are offset by approximately one-halfdetector pitch normal to the central ray of the incident x-ray, a stepsize of the sampling to profile 52 can be reduced from approximately dto approximately d/2 where d is the pitch size of the detector element18 while doubling the Nyquist frequency, thus resulting in an increasein resolution.

[0026] While the invention has been described in terms of variousspecific embodiments, those skilled in the art will recognize that theinvention can be practiced with modification within the spirit and scopeof the claims.

What is claimed is:
 1. A radiation detector, said radiation detectorcomprising: a first array comprising a first photon incident surface; asecond array comprising a second photon incident surface; and ascintillator array extending from said first photon incident surface tosaid second photon incident surface.
 2. A radiation detector inaccordance with claim 1 wherein said first array is offset from saidsecond array by approximately one-half detector pitch normal to anincident x-ray direction.
 3. A radiation detector in accordance withclaim 1 wherein said scintillator comprises a plurality of opticalfibers.
 4. A radiation detector in accordance with claim 1 wherein saidscintillator comprises a sheet of scintillator material.
 5. A radiationdetector in accordance with claim 1 wherein said scintillator array isconfigured to direct at least a portion of a plurality of opticalphotons to said first photon incident surface and said second photonincident surface.
 6. A radiation detector in accordance with claim 1wherein said first array and said second array comprises a plurality ofsensor elements comprising a plurality of photosensor devices.
 7. Aradiation detector in accordance with claim 6 wherein said plurality ofphotosensor devices are disposed in a linear array pattern, such thateach photosensor device in said sensor element is disposed adjacent atleast one other photosensor.
 8. A radiation detector in accordance withclaim 7 wherein said photosensor devices are aligned along a sensorelement axis corresponding to a longitudinal dimension of said sensorelement.
 9. A radiation detector in accordance with claim 3 wherein saidplurality of optical fibers are oriented orthogonally to a path of aplurality of x-rays passing through a collimator.
 10. A radiationdetector, said radiation detector comprising: a first array comprising afirst photon incident surface and a plurality of sensor elements havingan aperture pitch size; a second array comprising a second photonincident surface and a plurality of sensor elements having the aperturepitch size, said first array sensor elements offset from said secondarray sensor elements by approximately one-half the aperture pitch sizeto facilitate achieving an increased resolution of the sensor elements;and a scintillator array extending from said first photon incidentsurface to said second photon incident surface, said scintillator arrayis configured to direct at least a portion of a plurality of opticalphotons to said first photon incident surface and said second photonincident surface, said scintillator comprising a fiber opticscintillator having a plurality of optical fibers bundled in an arrayand disposed such that said x-rays are incident on said fiber opticscintillator substantially perpendicular to a respective optical axis ofsaid plurality of optical fibers, said fiber optic scintillator furtherbeing optically coupled to at least two of said sensor elements suchthat said sensor elements are disposed at both ends of the plurality ofoptical fibers
 11. A method for fabricating a radiation detector, saidmethod comprising: fabricating a first array including a first photonincident surface; fabricating a second array including a second photonincident surface; and positioning a scintillator array between the firstarray and the second array such that the scintillator extends from thefirst photon incident surface to the second photon incident surface. 12.A method in accordance with claim 11 wherein said fabricating a firstarray and a second array comprises positioning the first array one-halfdetector pitch offset from the second array normal to an incident x-raydirection.
 13. A method in accordance with claim 11 wherein saidpositioning a scintillator array comprises positioning a scintillatorarray including a plurality of optical fibers.
 14. A method inaccordance with claim 11 wherein said positioning a scintillator arraycomprises positioning a scintillator array including a sheet ofscintillator material.
 15. A method in accordance with claim 11 whereinsaid positioning a scintillator array further comprises positioning ascintillator array to direct at least a portion of a plurality ofoptical photons to said first photon incident surface and said secondphoton incident surface.
 16. A method in accordance with claim 11wherein said fabricating a first array and a second array comprisesfabricating a first array and a second array including a plurality ofphotosensor devices.
 17. A method in accordance with claim 16 whereinsaid fabricating a first array and a second array including a pluralityof photosensor devices comprises fabricating a first array and a secondarray including a plurality of photosensor devices disposed in a lineararray pattern, such that each photosensor device in said sensor elementis disposed adjacent at least one other photosensor.
 18. A method inaccordance with claim 17 wherein said fabricating a first array and asecond array including a plurality of photosensor devices comprisesfabricating a first array and a second array including a plurality ofphotosensor devices aligned along a sensor element axis corresponding toa longitudinal dimension of said sensor element.
 19. A method inaccordance with claim 13 wherein said positioning a scintillator arrayincluding a plurality of optical fibers comprises positioning ascintillator array including a plurality of optical fibers orientedorthogonally to a path of a plurality of x-rays passing through acollimator.
 20. A method for fabricating a radiation detector, saidmethod comprising: fabricating a first array including a first photonincident surface including a plurality of sensor elements including aplurality of photosensor devices; fabricating a second array including afirst photon incident surface including a plurality of sensor elementsincluding a plurality of photosensor devices; positioning a scintillatorarray between the first array and the second array such that thescintillator extends from the first photon incident surface to thesecond photon incident surface, the scintillator array is configured todirect at least a portion of a plurality of optical photons to the firstphoton incident surface and the second photon incident surface, thescintillator including a fiber optic scintillator including a pluralityof optical fibers bundled in an array and disposed such that the x-raysare incident on the fiber optic scintillator substantially perpendicularto a respective optical axis of the plurality of optical fibers, thefiber optic scintillator further being optically coupled to at least twoof the sensor elements such that the sensor elements are disposed atboth ends of the plurality of optical fibers.
 21. A projectionradiography imaging system imaging system for generating an image of anobject, said imaging system comprising: a detector array comprising: afirst array comprising a first detector pitch (d); and a second arraycomprising a second detector pitch (d) equivalent to said first detectorpitch, said second detector offset from said first detector byapproximately one-half detector pitch normal to an incident x-raydirection; one radiation source; and a computer coupled to said detectorarray and said radiation source, said computer configured to: samplesaid first array and said second array approximately simultaneously,combine the samples by interleaving the samples together to yield aneffective sampling step size of approximately one-half detector pitch;and reconstruct an image of the object using the interleaved first arraysamples and the second array samples.
 22. A radiography imaging systemimaging system in accordance with claim 21 wherein said detector furthercomprises a scintillator array extending from said first array to saidsecond array.
 23. A radiography imaging system in accordance with claim22 wherein said scintillator comprises a plurality of optical fibers.